Leak indicating missile finish



United States Patent F 3,368,994 LEAK INDICATING MISSILE FINISH ArthurStander, Philadelphia, Pa., assignor to the United States of America asrepresented by the Secretary of the Navy No Drawing. Continuation-impartof application Ser. No. 334,052, Dec. 27, 1963. This application Aug. 2,1966, Ser. No. 570,142

1 Claim. (Cl. 26031.8)

ABSTRACT OF THE DISCLOSURE The subject invention relates to an improvedcomposition which is applied to the exterior surface of a rocket motorfuel container and which detects leakage of the contents of thecontainer by means of a change in its color. The ingredients of theimproved composition include titanium dioxide, a resinous vehicle suchas a methyl methacrylate polymer, the color indicates phenolphthaleinand diphenylbenzidine which respectively change color in the presence ofan amine fuel and red fuming nitric acid, the plasticizer butyl benzylphthalate, and a suitable solvent.

The invention described herein may be manufactured and used by or forthe Government of the United States of America for governmental purposeswithout the payment of any royalties thereon or therefor.

This is a continuation-in-part of patent application, Ser. No. 334,052filed Dec. 27, 1963 and now abandoned.

This invention relates to an improved coating composition in the natureof a paint adapted to be applied to the exterior surface of liquid fuelpropelled aircraft launched missiles. More specifically, my coating byvirtue of inherent color changes is particularly effective in providinga means for visually identifying the location of a leak even though theliquid propellant may be escaping through only a minute apertureimperceptible to the naked eye.

A liquid propulsion system for a rocket motor consists of a fuelcomponent such as a mixed amine fuel and an oxidizing component such asred fuming nitric acid. The two liquids are stored in separate sectionswithin the motor. These sections are separated by suitable means toenable the two liquids to be brought into contact with one another toproduce a chemical reaction which detonates the motor to drive therocket. Because of the relatively high pressures involved and thecorrosive nature of the chemical products, leakage of the liquidcomponents in minute cracks in the motor encasement frequently occurs.Previously, there was no suitable means for immediately determiningwhether a leak had occurred either in the mixed amine fuel compartmentor in the red fuming nitric acid compartment of the rocket motor.

Accordingly, it is one object of the present invention to provide anindicator for selectively detecting the leakage of either the mixedamine fuel component or the red fuming nitric acid oxidizing componentfrom a liquid propulsion system which may be readily utilized withoutspecific treatment beforehand or development afterward and which will behighly stable regardless of varying temperature and humidity conditions,and upon leakage of either liquid component will give a trace or recordof desirable dark color and contrast which will have superiorpermanency.

Another object is to provide a coating composition which may be readilyapplied to a metal surface, which will be non-cracking and which willnot flake or peel off when the temperature of the surface exceeds 525 F.

3,368,994 Patented Feb. 13, 1968 A further object is to provide acoating composition which will resist any contamination by hydraulicfluids and which is effective in detecting propellant leaks throughminute openings.

Other objects and advantages will appear in the more detaileddescription set forth hereinbelow, it being understood, however, thatthis more detailed description is given by way of illustration andexplanation only, and

not by way of limitation, since various changes therein I may be made bythose skilled in the art without departing from the scope and spirit ofthe present invention.

In accordance with the present invention, there is provided acomposition of matter which, when applied as a coating on the rocketmotor imparts a uniform white finish thereto. Where a leak occurs, themixed amine fuel or the red fuming nitric acid causes a substance havinga color differing from the uniform white shade of the originalcomposition to be deposited on the surface of the coating, therebyproviding a distinctly contrasting color or shade at the point ofleakage. The deposited substance forms colored rings or spotsimmediately over the leak and remains there permanently until removed sothat an examination for leakage can be made at any time. The coating isnot affected by high humidity nor by temperatures well in excess of 400F., i.e., the constituents of the coating retain their stability at suchelevated temperatures.

I have proposed to incorporate a combination of colorless indicatorsconsisting of diphenylbenzidine and phenolphthalein into a plasticizedacrylic vehicle wherein the diphenylbenzidine turns blue when contactedby red fuming nitric acid and the phenolphthalein turns red whencontacted by the mixed amine fuel. The idea is simple but it has notbeen satisfactorily accomplished in the past due to the difliculty ofincorporating these indicators into a suitable paint vehicle. Thus, forexample, when these indicators were incorporated into an alkyd vehicle,the finish would not dry properly. 'Polyamide-epoxy combinations turnpink soon after application, polyurethanes gel on storage andnitrocellulose containing finishes darken soon after application.

In a preferred form of the present invention, there is provided anintimate mixture of a material such as powdered titanium dioxide,phenolphthalein, diphenylbenzidine, and an acrylic vehicle incorporatingbutyl benzyl phthalate as a plasticizer therefor. The acrylic vehicle isone selected from the group consisting of methyl methacrylate polymer,ethyl methacrylate polymer and methyl N-butyl methacrylate polymer, eachof which is resistant to hydraulic oils. The color of the resultingmixture is white because of the presence of the titanium dioxide. Toenable application of the composition to the surface of the missilebeing leak tested by conventional methods such as brushing, dipping orspraying, a solvent, being one selected from the group consisting oftoluene, xylene and butanol is added to the mixture to form a suspensionthereof, the amount of the solvent depending upon the consistencydesired in the suspension. When the solvent suspension of the mixture isapplied to the surface, the solvent evaporates and an adherent coatingof the mixture remains on the surface. Although the proportions of theconstituents of the composition of the present invention are notcritical, it is desirable that the constituents be present in themixture in the range of 10-20 parts by weight of titanium dioxide, 50-75parts by weight of the acrylic vehicle, 4-7 parts by weight of butylbenzyl phthalate, 3-5 parts by weight of phenolphthalein and 0.05-0.02.parts by weight of diphenylbenzidine. The amount of solvent added toform the suspension may be 20-50 parts by weight.

Within the foregoing ranges of constituents, one mixture which gaveexcellent results comprised parts by weight of titanium dioxide, 4 partsby weight of butyl benzyl phthalate and 65 parts by weight of methylmethacrylate polymer, the molecular weight of said' polymer being suchthat a 30% solution of said polymer in toluene has a viscosity of235-365 centiposes at 30 C. The mixture is readily accomplished byintimately mixing the constituents in a mortar or ball mill. The mixtureis then agitated in about 35 parts by weight of toluene to form thesuspension.

This coating is sprayed over the surface of a missile. It will dryquickly to a gloss white finish. A leak of the mixed amine fuelcomponent causes the finish to turn red and a leak of the red fumingnitric acid causes the finish to turn blue.

Obviously, many modifications and variations of the present inventionare possible in the light of the above teachings. It is therefore to beunderstood that within the scope of the appended claim, the inventionmay be practiced otherwise than as specifically described.

I claim:

1. A composition for application as a permanent coating to the exteriorsurface of a rocket motor containing a mixed amine fuel component and ared fuming nitric acid oxidizing component and through which surfacesaid amine fuel and said red fuming nitric acid may leak, which whenapplied as a coating and dried will be resistant to hydraulic fluids,high atmospheric humidity and temperatures to which it may be exposed,and which 4 when so applied will clearly indicate the existence andlocation of any such leak that may subsequently develop, which consistsessentially of the following constituents.

Constituent: Parts by Weight Phenolphthalein 3-5 Diphenylbenzidine0.O50.2 Titanium dioxide 10-20 Butyl benzyl phthalate 47 A methylmethacrylate polymer having a molecular weight such that a solution ofthe polymer in toluene has a viscosity of 235-365 centipoises at 30 C.50-75 Toluene 20-25 References Cited UNITED STATES PATENTS 2,845,3947/1958 Thompson 252-408 2,934,510 4/1960 Crissey et al. 260-31.82,943,475 7/1960 Benveniste et al. 260

OTHER REFERENCES Horn: Acrylic Resins; Reinhold Publishing Corp., 1960,pp. 142, 143.

Rosin: Reagent Chemicals and Standards; D. Van Nostrand Company, Inc.;4th ed., 1961, pp. 165, 300, 301.

MORRIS LIEBMAN, Primary Examiner.

L. T. JACOBS, Assistant Examiner.

